1. Field of the Invention
The present invention relates to detection of a jam at a fixing device of an image forming apparatus.
2. Description of the Related Art
In an image forming apparatus, such as a copying machine or a printer, a sheet detecting sensor is provided at a sheet conveying path to determine whether or not a sheet jam has occurred from a detection result. A “winding jam” is one type of jam in which a front edge of a sheet that has passed a roller in a fixing device is not separated from the roller in the fixing device and, as a result, is wound upon the roller in the fixing device. When the sheet is completely wound upon the fixing device due to the winding jam, toner on the sheet causes the roller in the fixing device to adhere to the sheet. Accordingly, it takes a lot of time and trouble to separate the sheet from the roller. For instance, this may make it necessary for service personnel to disassemble and repair or replace the fixing device.
Therefore, there are proposed image forming apparatuses (such as those disclosed in Japanese Patent Laid-Open Nos. 7-140836 and 2003-323068) that are capable of stopping a fixing device when a sensor, disposed at an exit side of the fixing device, does not detect a sheet at a predetermined timing and it is, thus, determined that the sheet is wound upon a fixing roller. These image forming apparatuses make it possible for a user to properly perform operations as a result of stopping the apparatuses before the sheet is completely wound upon the fixing device.
In a high-speed copying machine, such as a copying machine that prints 100 pages of A4 size sheets every minute, even if driving of a fixing device is stopped when a winding jam occurs, the fixing device cannot be stopped immediately due to inertia of a fixing roller and a motor. Therefore, it is not possible to prevent a sheet, such as an A4 size sheet or an LTR size sheet, having a short length in a conveying direction from being completely wound upon the fixing roller.
Accordingly, apparatuses, such as those disclosed in US patents, having the following feature are provided. Such apparatuses make it possible to stop a fixing motor before even a small sheet is completely wound upon a fixing roller, as a result of suddenly braking the fixing motor by passing an electrical current that stops driving the fixing motor in a forward direction and that, at the same, rotates the fixing motor in a reverse direction when stopping the driving of the fixing motor.
However, in the above-described related image forming apparatuses, when sudden braking is performed whenever the driving of the fixing device is to be stopped, a large load is applied to the fixing motor each time the fixing motor is rotated and stopped. As a result, the life of the fixing motor is reduced, thereby reducing exchange cycles of the fixing device, and thus, increasing operational costs.